The present invention generally relates to an inverter-driven motor for industrial use, which is driven by an inverter power source and more particularly, to an inverter-driven motor of a variable speed electric compressor which is built into an air-conditioner or the like, contains refrigerant and freezed machine oil and is driven by an inverter power source.
In recent industrial fields, a trend that motors, for example, motors for driving cutting machine tools, are driven by inverters is gaining momentum because the speed of the inverter-driven motors can be changed arbitrarily. Meanwhile, also in the field of air-conditioners, the number of air conditioning systems employing electric compressors driven by inverter power sources is on the increase. Use of the inverter power sources leads to more a comfortable air conditioning state and enables change of capabilities of the air conditioning systems, saving of energy and silent running. Demand for these features of the inverter power sources is becoming greater in the market.
However, if a motor is driven by an inverter power source, such a problem arises that quantity of leakage current is greater than that of a case in which a commercial power source is used. FIG. 11A shows wave form of output voltage of the inverter power source, while FIG. 11B shows wave form of output voltage of the commercial power source. FIG. 12A shows wave form of output current of the inverter power source, while FIG. 12B shows wave form of output current of the commercial power source. Wave form of the inverter power source contains more harmonic components than those of the commercial power source. On the other hand, an insulating film for a groove insulation in the motor has capacitance and its impedance Z is expressed by the following equation (1): EQU Z=1/(2.pi.fC) (1)
where "f" denotes frequency and "C" denotes capacitance.
In case the inverter power source is used, the frequency f rises, so that the impedance Z decreases due to the many harmonic components as is apparent from the above equation and thus, electric current readily leaks from a winding wire to a grounded iron core via the capacitance.
FIG. 9 is explanatory of a method of measuring leakage current of an inverter-driven motor 28 of a known electric compressor 40 driven by an inverter power source 25 and having a compression mechanism 22. The inverter-driven motor 28 is built in a casing 21 of the electric compressor 40 and mixed liquid 23 of refrigerant and freezed machine oil are contained in the casing 21. In FIG. 9, voltage from the inverter power source 25 is applied to a winding wire 5 of the inverter-driven motor 28 through a terminal 24 insulated from ground. At a groove 6 for holding the winding wire 5, insulation between the winding wire 5 and an iron core 11 is performed by an insulating film 7 of several hundreds .mu.m in thickness and a wedge type insulating film 8 having a thickness of several hundreds .mu.m. Electric current having harmonic components leaks from the winding wire 5 to the iron core 11 grounded via capacitance and DC insulation resistance of the insulating film 7 and the wedge type insulating film 8 and is measured by a leakage current meter 27.
FIG. 10 shows an equivalent circuit of a flow path of the leakage current of FIG. 9. In FIG. 10, electrical function of the insulating film 7 in the groove 6 is divided into a capacitance 29 and a DC insulation resistance 30.
Conventionally, in order to reduce leakage current, a first method in which the diameter of the winding wire 5 is reduced and a second method in which thickness of the insulating film 7 is increased have been employed. In the first method, since the capacitance 29 of FIG. 10 is lessened by reducing diameter of the winding wire 5, harmonic components of leakage current are less likely to flow. Here, a capacitance C of a capacitor has the following relation (2): EQU C=.di-elect cons.S/d (2)
where ".di-elect cons." denotes dielectric constant, "S" denotes area of each of a pair of electrode plates of the capacitor and "d" denotes distance between the electrode plates. Assuming that ".di-elect cons..sub.o " denotes spatial dielectric constant and ".di-elect cons..sub.s " denotes relative dielectric constant, the dielectric constant .di-elect cons. is a product of the spatial electric constant .di-elect cons..sub.o and the relative dielectric constant .di-elect cons..sub.s, namely, .di-elect cons.=.di-elect cons..sub.o .di-elect cons..sub.s.
According to the equation (2), the capacitance C decreases as the area S of each of the electrode plates is reduced. In the inverter-driven motor 28 of FIG. 9, each of the surface area of the groove 6 and the surface area of the winding wire 5 held in the groove 6 corresponds to the area S of each of the electrode plates. Therefore, reduction of the surface area of the winding wire 5 by reducing the winding wire diameter results in reduction of the area S of each of the electrode plates of the capacitor, so that the capacitance 29 can be lessened and thus, the impedance Z increases based on the equation (1). Consequently, since harmonic components of leakage current are less likely to flow, quantity of leakage current can be reduced effectively.
However, if the surface area of the winding wire 5 is lessened by reducing the winding wire diameter, resistance value of the winding wire 5 increases, so that Joule loss of electric current increases and thus, primary copper loss of the motor increases, thereby resulting in a drop in the efficiency of the motor.
In the above mentioned second method, since the capacitance 29 of FIG. 10 is lessened by increasing thickness of the insulating film 7, harmonic components of leakage current are less likely to flow. FIG. 8 shows one example of a known insulating film 7 which is formed by insulating films 7a and 7b such that thickness of the insulating film 7 is increased.
In the equation (2), if the distance d between the electrode plates of the capacitor is increased, the capacitance C decreases. In the inverter-driven motor 28 of FIG. 9, thickness of the insulating film 7 corresponds to the distance d between the electrode plates of the capacitor. Therefore, since the capacitance 29 can be lessened by increasing thickness of the insulating film 7, harmonic components of leakage current are less likely to flow, thus resulting in reduction of quantity of leakage current.
However, if thickness of the insulating film 7 is increased, the effective area for holding the winding wire 5 in the groove 6 is reduced and thus, the whole of the winding wire 5 cannot be accommodated in the groove 6. Thus, if the diameter of the winding wire 5 is reduced so as to lessen the sectional area of the winding wire 5, the whole of the winding wire 5 can be accommodated in the groove 6. However, in this case, since resistance value of the winding wire 5 increases as described above in connection with the first method, efficiency of the motor drops undesirably.